As a thermistor, a PTC (Positive Temperature Coefficient) thermistor with a positive temperature coefficient of resistance α has been known. The PTC thermistor will have an increase in the resistance against the increase of the temperature, thus it can be used as a heater, an over current protection element, an overheat detection sensor and the like. In the prior art, a PTC thermistor has been obtained by adding a minute amount of rear earth elements and the like to barium titanate (BaTiO3) which is the main component and forming it to a semiconductor. Therefore, it will have a sharp increase in the resistance by several orders of magnitude above the Curie temperature, while it has a low resistance under Curie temperature.
The Curie temperature of BaTiO3 is usually 120° C. However, it can be shifted to a lower temperature by substituting a part of Ba with Sr or Sn. As for the shift of the Curie temperature to be higher, it has been realized by substituting a part of Ba with Pb at present. From the view point of the trend of decreasing the environmental load of the world, practical application of a substitute material with no Pb has been demanded.
In the following Patent Document 1, a method for producing a semiconductor ceramic composition has been disclosed. In the method, one or more of any of Nb, Ta or a rare earth element are added into a semiconductor ceramic composition consisting of Ba1-2X(BiNa)XTiO3 (0<X≦0.15), in which a part of Ba is substituted with (Bi, Na) rather than Pb. Then, after the composition is sintered in nitrogen, it is heat-treated in an oxidation atmosphere.
In addition, in the following Patent Document 2, a method for producing a semiconductor ceramic composition has been disclosed. In the method, the sintered body of a semiconductor ceramic composition in which a part of Ba of BaTiO3 is substituted with (Bi, Na), is applied to a heat-treatment under a temperature below 600° C. in air atmosphere after electrodes are formed on it, as a means to increase the change ratio of the increased resistance above the Curie temperature to the specific resistance at a normal temperature (herein after, referred as “temperature coefficient of resistance α”).
Further, in the following Patent Document 3, a semiconductor ceramic composition without Pb has been disclosed. The composition is produced by preparing BT calcined powder consisting of (BaR)TiO3 (wherein, R is at least one of rare earth elements) calcined powder or Ba(TiM)O3 (wherein, M is at least one of Nb or Sb) calcined powder, and BNT calcined powder consisting of (BiNa)TiO3 calcined powder, respectively, sintering the molded body prepared from the mixed calcined powders of the BT calcined powder and the BNT calcined powder in an atmosphere containing 1 vol % or less of oxygen, then applying the sintered body to a heat-treatment for 0.5 hours or more and 24 hours or less under a temperature of 300° C. or more and 600° C. or less in an atmosphere containing 0.1 vol % or more of hydrogen.
All of the above mentioned Patent documents have disclosed that, a semiconductor ceramic composition without using Pb, which has a Curie temperature shifted to a temperature higher than 120° C., a small specific resistance at a normal temperature, and a large temperature coefficient of resistance α, can be obtained.